This invention relates to a circuit for charging a battery of a battery operated appliance. More particularly, this invention is directed toward a circuit for operating and charging a battery of an appliance with a constant effective current from an AC power source which may be of a plurality of line voltages.
With portable electric and electronic devices, it is desirable that they can be operated from different voltages and, if they have storage batteries, that these storage batteries can be charged at different voltages. For example, electric razors, flash units, portable radios, and the like are often taken along on journeys to foreign countries and are then operated at the different line voltages in the respective countries. As a rule, these line voltages vary on the one hand between 110 volts and 240 volts and on the other hand between 50 Hz and 60 Hz.
Numerous circuit arrangements have already been proposed for matching the small electric appliances to various line voltages. For example, a circuit arrangement is known for the capacitive transformation of an input voltage. By means of this arrangement, it is possible to operate a small electrical appliance at various D.C. and A.C. currents within a prescribed range (DE-OS 26 14 746). However, with this circuit arrangement, there is no means which is especially intended for charging a storage battery or a nickel-cadmium battery.
A circuit arrangement is also known by means of which an electric razor can be operated over a wide range of different input A.C. voltages (U.S. Pat. No. 4,001,668). With this circuit, a capacitor is charged through transistors, in pulse-fashion, to a particular voltage, by means of which a motor or the like is driven. Special means for charging a storage battery are likewise not provided by this circuit arrangement.
A circuit, in which a capacitor is charged up to a prescribed value, is also known from U.S. Pat. No. 4,080,646. Here, a chopper circuit is provided, which is shut off when the prescribed capacitor voltage is reached. A special charging circuit for storage batteries or for nickel-cadmium cells, however, is not contained in this circuit arrangement.
Special charging circuits for rechargeable batteries or the like have already also been proposed. With a known charging circuit, a thyristor is provided which passes or blocks the charge current depending on the voltage existing at the battery (U.S. Pat. No. 4,081,729). This charging circuit, however, is not suitable for different input line voltages.
Other charging circuits have been proposed for charging nickel-cadmium cells and for use with electric razors. They contain a high-frequency oscillator which is connected to the line voltage, and which is dependent on the charge state of the nickel cadmium cells (U.S. Pat. No. 4,128,798, DE-OS No. 24 02 182). These charging circuits likewise cannot be used with various line voltages.
A battery charging unit, which can be used with various input A.C. line voltages, has been proposed (U.S. Pat. No. 3,943,423). This charging unit contains a controllable transistor and a current amplifier. Only negative feedback is provided which couples the output of the current amplifier back to the control electrode of the transistor. In this way, a higher charging current flows when the input voltages are lower.
A constant-current charging unit for various line voltages is also known. This circuit is intended for the continuous charging of nickel-cadmium mini-storage batteries, and includes a transformer (DE-AS No. 21 37 883). With this charging unit, the transformer is designed as a current transformer. The primary coil of the transformer contacts the line through a capacitor, while the secondary coil of the transformer feeds through a rectifier to the battery being charged.
Finally, a transistor-transformer circuit is also known, by means of which it is possible to transform an A.C. line voltage either into a low D.C. current for charging a battery or into a high D.C. current for directly driving a D.C. motor (DE-OS No. 20 14 377). With this transformer circuit, a high-frequency oscillator with a saturable core is provided. This circuit contains two mutually coupled coils, one of which is in series with a switching transistor.